In order to guarantee the safety and the stability of a power system, Chinese grid construction is planned and configured according to three defense lines, and grid safety operation is also scheduled and managed according to the three defense lines. Chinese power systems have been rapidly developed in recent years, mass introduction of intermittent renewable energy power generation ways such as wind power generation, photovoltaic power generation and the like increases the complexity of grid characteristics, and they are substantially un-schedulable and equivalent to random disturbance sources, greatly influence the reliability operation of grids, and increase the grid scheduling difficulty. With the construction of smart grids, the requirement of users for power supply quality also rises. Planning and constructing the grids based on the traditional three defense lines cannot well meet the construction requirements. Particularly, the low-frequency load reduction control measure in the third defense line not only seriously influences power supply of users, but also may remove part of important loads, resulting in major economic loss and safety accidents, so low-frequency load reduction should be avoided as much as possible.
Low-frequency load reduction: in order to prevent frequency collapse of a power system, when the frequency of the power system declines due to vacancy between power generation and the demand of electrical loads, part of secondary loads prearranged in the system are cut off successively according to a preset action frequency value, so that the active power of the system regains a tendency of balance and the frequency rises. In China, the low-frequency load reduction action is generally divided into 5˜6 turns, the starting frequency of the first turn is set at 48.5˜49.1 Hz, and the frequency of the last turn is 47.0˜47.5 Hz.
In order to avoid the occurrence of unnecessary low-frequency load reduction, part of unimportant loads can be cut off before the low-frequency load reduction action caused by the degree of frequency decline, so that the vacancy of active power is reduced, the frequency stability of the system is improved, the low-frequency load reduction is avoided, and the economical loss is reduced to minimum.
Electric vehicle charging loads are ideal controllable loads. When the grid frequency declines greatly after fault, part of the electric vehicle charging loads can be cut off, to temporarily stop charging thereof and continue charging after the frequency is restored. In this way, not only can the low-frequency load reduction be effectively prevented, but also charging of an electric vehicle is not affected greatly.
At present, electric vehicle charging modes are divided into three modes, a slow charging mode, a fast charging mode and a battery replacing mode. Electric vehicle charging piles are divided into three types, an alternating-current charging pile and a direct-current charging pile, the alternating-current charging pile provides slow charging for electric vehicles, and the direct-current charging pile provides fast charging for electric vehicles and is applied most widely. Moreover, the output power of the direct-current charging pile is adjustable, multi-level power output can be achieved, and each level of power can be nearly seamlessly switched on line.
Hence, the charging pile is a controllable load for grids, its load power may be high or low, and reducing the output power of the charging pile is equivalent to cutting off part of charging loads, has the same effect as the low-frequency load reduction, but has little influence on users, and does not influence normal operation of loads.
Thus, the change of grid frequency is monitored in real time, and when the frequency declines greatly, the output power of the charging pile can be reduced or even charging is cut off in order to avoid the low-frequency load reduction action, so that the power difference of the power system is reduced, adjusting time is provided for the adjustment of the power system with a large time constant, and the frequency stability of the power system is improved. Moreover, the more the on-line charging electric vehicles are, the higher the adjustable charging load is, and the more obvious the above adjusting effect is.
Accordingly, it is quite necessary to design an electric vehicle charging pile control system considering grid frequency safety.